1. Field of the Invention
This invention relates to the field of infrared sensing, and more particularly concerns a method and apparatus for detecting infrared radiation.
2. Description of Related Art
Elemental infrared detectors are often used in surveillance, target seeking, and search and tracking imaging systems to sense the presence of electromagnetic radiation having wavelengths from 1-30 .mu.m. To detect infrared radiation, these elemental detectors often use temperature sensitive pyroelectric and ferroelectric materials such as triglicine sulfate and lanthanum doped lead zirconate titanate crystals. Such crystals exhibit spontaneous electrical polarization in response to incident infrared radiation which creates a potential drop across electrodes attached to the crystals. Photoconductive materials such as lead-sulfide and mercury-cadmium-telluride may also be used in which the resistance of the material changes as a function of incident radiation. Finally, photovoltaic devices such as those fabricated from mercury-cadmium-telluride, indium antimonide, or similar materials may be used in which intrinsic band-to-band electron-hole excitation generates a current or voltage which is proportional to the incident radiation flux.
Arrays of such elemental detectors may be used to form thermal imaging systems. In real-time thermal imaging systems such as forward looking infrared ("FLIR") imaging sensors, oscillating prism mirrors are used to scan radiation emitted by a source across a one-dimensional array of elemental detectors. When the elemental detectors are used in this manner, the temporal outputs of the detectors may be used to generate a two-dimensional representation of the image. In two-dimensional detector array imaging systems which can utilize either staring or scanning arrays, the elemental detectors produce free charge carriers or currents which may be monitored by an appropriate readout integrated circuit such as a charge-coupled device ("CCD"). The output from the CCD can be processed by various techniques such as time delay and integration or parallel-to-serial scan conversion, with the choice depending on the system requirements of frame rate, signal-to-noise ratios, etc. Other readout devices may also be used.
While the detector structures described above are effective, they generally have several drawbacks in terms of fabrication. First, many of such detector structures are hybridized in which the detector and the readout device are separately fabricated and then mechanically bonded. Because the formation of hybridized structures involves additional processing steps, such structures tend to have lower yield during production when compared to monolithic devices (i.e., devices in which the detector and the readout and signal processing circuit are fabricated in one material system). In addition, there is a greater likelihood that defects while be unintentionally introduced during fabrication resulting in lower reliability and reduced performance. Monolithic detectors, while avoiding the additional processing steps required for hybridization, suffer from the necessity of using the same material as the infrared detector for the signal processing circuits. This material (e.g., HgCdTe or NnSb) invariably leads to lower performance signal processing circuits than circuits formed from silicon or gallium arsenide, for which the fabrication technology is well-developed.